There are numerous optical methods for finding optically visible defects, such as holes or spots in industrial material sheets. Manufacturers of such strip or web materials use optical inspection and measurement systems e.g. for controlling the manufacturing process of the materials in order to improve yield in terms of improved quality, decreased waste and machine down time on the manufacturing line.
The optical measurement systems referred here typically operate online i.e. simultaneously with the product manufacturing and are non-contacting.
Detection of such quality defects as pinholes, holes, spots, scratches, streaks, cracks, cuts, tears or edge defects are important applications where aforementioned optical inspection and measurement systems may be used. Defects or material properties of other kinds may also be measured with the described systems. Web or strip sheet width, length or edge position measurements, are other examples of the possible uses of these systems.
One of the methods in accordance with prior art utilises CCD (Charge Coupled Device) cameras. The operation of a CCD camera cell can be divided into two distinctive periods: the integration period and the readout period. During the integration period the cell is active in terms of light intensity measurement and during the readout period it is not. In a typical CCD camera system each CCD camera cell records the integrated light intensity falling upon it during a certain integration period. The resulting integrated electrical charge is stored in each CCD cell until the cell is read out. Typically the total electric charge generated by the photons is stored in a capacitor in each pixel. As a photon hits a pixel, a small amount of charge is added to the capacitor. This process is called the integration period of the device.
The integration period continues until a certain time has elapsed, and after the completion of the integration, readout period starts. In the readout phase the charge proportional to the incident photon number is observed and recorded, and thus incident photon number can be deduced with certain accuracy. After the readout is complete, the CCD is flushed from the stored charges and a new integration period starts.
For example the ULMA product range from ABB Corporation has utilised CCD cameras in web inspection, please see “ULMA Nti tuote data” product publications for reference from ABB. Earlier ULMA products have also utilized phototransistors generating photocurrent.
FIG. 1 shows a flow diagram explaining the prior art. In phase 101 a material sheet is stationary or is traversed between and/or in front of one or more optical light sources and light detectors. In phase 111 a light source, or several emit light beams and shine the beams on a material sheet. In phase 121 light beam targeted towards the material interacts with the material sheet to be inspected or measured.
In phase 131 light is detected at a light detector or light detectors. The light detector or light detectors convert incident light into photocurrent signals in phase 141. In phase 151 the photocurrent signal is processed and manipulated to determine characteristics of the material. Prior art solutions of this type are found for example from GB 2181834 and GB 2087544 which are cited as reference.
Photo multiplier tubes (PMTs) are also used for inspection and measurement of defects in materials manufactured in a continuous fashion. PMTs are most typically used for detection of pinholes in materials. Holes or pinholes in a material sheet may be detected by using a UV (Ultraviolet) light source or a scanning, laser light source on one side of a material and one or several PMTs on the other side. In this case the PMT or PMTs are used to detect the UV or laser light transmitted through the hole while the material traverses the measurement system.
There are several inherent disadvantages in the prior art. The prior art method of FIG. 1 is prone to ambient light, both optical and electrical noise and the level on signal strength is typically also a problem.
The CCD devices are integrating and imaging devices; there are strict limits on the speed of detection. If the material is traversed faster, the CCD equipment may be unable to photograph the whole surface area of the sheets, due to the latency in integration and image readout. The integration method CCD cameras are based upon is incremental, not continuous, and therefore undesirably slow and unreliable. The integration periods of CCDs are also typically quite long for the purposes of dynamical defect detection.
CCD systems also typically operate with visible wavelengths, and ambient light is therefore a problem. A significant disadvantage of the prior art is that either the system has to be covered from ambient light, or it must bear the errors caused by ambient light. Optical filtering is typically inefficient, as the measurements are done at the same wavelengths as ambient light.
CCD camera systems are imaging systems that produce photograph like, digital images of the material to be inspected or measured. All the image information produced by the CCD camera must typically first be stored in specialized image processing electronics or in computer memory and then transferred and/or analysed in a computer system to distinguish useful measurement information from all unnecessary information. The CCD camera itself cannot discriminate and select inspection or measurement data useful for the user of the system. Especially in large industrial inspection and measurement systems, extensive data storage, transfer and computing capacity is therefore required. In many factories or industrial facilities computer systems of this scale are very expensive and tedious to arrange.
PMTs are mechanically vulnerable and measurement systems based on PMTs are poor in terms of shock or vibration resistance. UV light based PMT systems are also notoriously unstable, as the UV-source lifetime is typically only 1-2 months. Despite basically different wavelengths of the system light source and ambient light, PMTs are also sensitive to ambient wavelengths and ambient light remains a problem. In the edge area of the material under inspection, separate, mechanical edge following light shields must be used along the sides of the material, to prevent the PMTs located at the edge of the material from becoming saturated and therefore non-operational. The mechanical edge following shields are unreliable since these light shields need to be mechanically moved in demanding industrial environments with possible harmful interference with the material to be inspected or measured.
Any moving parts or parts mechanically interacting with the material to be inspected or measured are undesirable because of reliability reasons. For example, the edge followers are prone to cause measurement errors as they are subject to mechanical shear, strain and stress, and may typically move to destroy the calibrations of the delicate measurement system. Design of PMT based UV inspection systems for wide material sheets is quite unpractical due to the extensive demands set on mechanical engineering and high cost.
For clarification the opportunity is taken to define the following terms:
“Light receiver” and “light detector” are used in this application interchangeably. “Light detector” refers with emphasis to the semiconductor part of the light receiving detector and its associated optical, mechanical and electronic parts. “Light receiver” refers foremost to the entire optical, mechanical and electrical arrangement for receiving the light and comprises at least one light detector.
“Synchronisation signal” is a signal that is used to synchronise an emitter and a receiver with respect to waveform, phase and/or frequency of the signal.